Experimental and theoretical identification of adenine monolayers on Ag-terminated Si(111)

We discuss the formation of ordered adenine monolayers on a Ag-Si (111)root 3 x root 3R30 degrees surface held at room temperature. Adenine forms a hexagonal network, which is stabilized by hydrogen bonding. By comparing the ordering observed using scanning tunneling microscopy with molecular superstructures calculated using ab initio density functional theory, we suggest that adenine forms an array of irregular hexagons through the combination of three distinct hydrogen-bonded adenine dimers. The dimensions of the ordered monolayer are commensurate with the surface with root 3 x root 7 periodicity relative to the Ag-Si (111) root 3 x root 3R30 degrees reconstruction, in agreement with calculated molecular dimensions. An analysis of double-domain regions leads us to conclude that there are two domains with different chiralities.